1. Field of the Invention
The present invention is directed to a process for producing a phosphor layer on a substrate, suitable for producing a scintillator.
X-ray radiation can be converted into visible light with the aid of scintillators. This visible light can then be registered by photodetectors. Such scintillators are used, for example, for producing medical radiographs.
2. Description of the Prior Art
A phosphor layer which is disposed on a glass substrate is often used as a scintillator. In addition to a good light yield and a narrowly defined particle size and layer thickness of the phosphor layer, a good adhesion of the phosphor layer to the substrate is also important.
To measure the intensity distribution of a wide X-ray beam, for example for producing medical radiographs, a positional resolution is necessary in the plane of the intensity distribution to be measured. For this purpose, small scintillators which are disposed alongside one another are generally used. In the direction of the radiation propagation of the X-ray radiation, said scintillators have to be so thick that they protect the photodetectors against radiation damage by absorbing the X-ray radiation.
It is known to produce phosphor layers on a substrate by vapor deposition, screen printing or sedimentation of phosphors. The technical complexity and the susceptibility to defects of this process is generally high. A good adhesion of the phosphor layer to the substrate can only be achieved if the substrate is carefully cleaned. The phosphor is applied as powder. The powder already has the chemical composition necessary for the phosphor layer. This powder has to be produced beforehand in a process which is usually complex. In order to achieve a uniform layer thickness of the phosphor, the substrate must if possible have no profile (i.e., the substrate must be as smooth as possible).
It is known to produce scintillators in the form of layers composed of crystal needles disposed perpendicularly to the layer. The crystal needles comprise, for example, CsI:Na. In this case, the needles must have large enough dimensions in the direction of the incident X-ray radiation for the X-ray light to be largely absorbed. The positional resolution is determined by the diameter of the needles perpendicular to the direction of incidence of the X-ray radiation. In these scintillators, most of the X-ray light is converted into luminescent light in the first third of the scintillator crystal. In the rest of the crystal, the still unabsorbed radiation is attenuated to such an extent that the photodetectors are protected against radiation damage. The layers produced must therefore be disproportionately thick, compared with their light yield.